Sensor system for buried waste containment sites

ABSTRACT

A sensor system for a buried waste containment site having a bottom wall barrier and/or sidewall barriers, for containing hazardous waste. The sensor system includes one or more sensor devices disposed in one or more of the barriers for detecting a physical parameter either of the barrier itself or of the physical condition of the surrounding soils and buried waste, and for producing a signal representing the physical parameter detected. Also included is a signal processor for receiving signals produced by the sensor device and for developing information identifying the physical parameter detected, either for sounding an alarm, displaying a graphic representation of a physical parameter detected on a viewing screen and/or a hard copy printout. The sensor devices may be deployed in or adjacent the barriers at the same time the barriers are deployed and may be adapted to detect strain or cracking in the barriers, leakage of radiation through the barriers, the presence and leaking through the barriers of volatile organic compounds, or similar physical conditions.

RELATED APPLICATION

This application is a Divisional of allowed U.S. application Ser. No.09/418,681, filed on Oct. 14, 1999 now U.S. Pat. No. 6,648,552.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has certain rights in this inventionpursuant to Contract No. DE-AC07-94ID13223, DE-AC07-99ID13727, andContract No. DE-AC07-05ID14517 between the United States Department ofEnergy and Battella Energy Alliance, LLC.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a sensor system formonitoring the structural integrity of an underground waste containmentbarrier, and leakage therefrom of waste products or byproducts, and forimproved characterization of zones of interest.

2. Background Art

It is often necessary to form a containment barrier around a hazardouswaste site to stop or prevent the migration of contaminants into thenearby soil and water tables. The containment barrier must prevent themigration of contaminants both horizontally and vertically away from thewaste site. Therefore, a properly constructed containment barrier may becompared to a huge bathtub, with the hazardous waste contained withinfour side walls and a generally horizontal floor.

A typical, currently-used method of containment is to physically removethe hazardous waste and haul it to a permitted storage facility.However, such method is costly, impractical, and dangerous. Digging upsites with buried drums, radioactive dusts, or other airborne wastes mayactually release the contaminants, spreading them into the atmosphereand through the soil.

In response to this problem, a number of suggestions have been made forplacing containment barriers around hazardous waste sites, withoutremoving the waste. One approach for doing this is disclosed inInternational Publication Nos. WO 94/19547 and WO 93/00483 byHalliburton Nus Environmental Corp. The Halliburton system uses a row ofhigh pressure jets to shoot a slurry into soil surrounding a hazardouswaste site, somewhat liquefying the surrounding soil. The slurry cuts apath through the soil as it intermixes with the liquified soil. Gravityand/or mechanical means pull the row of high pressure jets through themix of liquified soil and slurry, after which the liquified soil andslurry harden into a protective barrier.

The above-described system has a number of shortcomings, including thepossibility of further spreading contaminants by the use of hydraulicjets, the difficulty of maintaining balance between the amount of slurryneeded for cutting and the amount of slurry needed for hardening thesoil, the difficulty of providing a barrier of consistent strength sinceit would depend in part upon the soil composition encountered and theamount of slurry deposited, and, finally, the lack of testing ofexcavated soil to know whether soil surrounding the waste site hasbecome contaminated.

Another suggested approach for installing a containment barrier around ahazardous waste site is disclosed in patent application Ser. No.08/925,101, filed Sep. 8, 1997, now U.S. Pat. No. 6,016,714 issued Jan.25, 2000. In this approach, a multi-layer containment barrier is put inplace under a hazardous waste site without disturbing any buried waste,in a simple and efficient fashion. The disclosure in the above-notedco-pending patent application is incorporated herein by reference.

In any approach to holding hazardous waste, it would be desirable tomonitor the site in terms of both the structural integrity of anycontainment barrier put in place about the waste material, and leakageof contaminants away from the site. Additionally, it would be desirableto monitor material being excavated from around a waste site inpreparation for emplacement of a containment barrier for the site, todetermine the extent of contamination of surrounding soils and thus thepossible need to extend the containment barrier to a location completelysurrounding all contaminated materials and soils. Finally, it would bedesirable to efficiently and inexpensively install a long-termmonitoring system soon after or simultaneously with the installation ofthe containment barrier.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a sensor system for sensinga variety of physical parameters of a buried waste containment site.

It is also an object of the invention to provide such a sensor systemespecially suitable for use in connection with a containment barrierdisposed under and around a buried waste site.

It is a further object of the invention to provide such a sensor systemfor monitoring the structural integrity of such a containment barrier.

It is also an object of the invention to provide such a sensor systemfor sensing leakage of contaminants from a buried waste containmentsite.

It is still another object of the invention to provide such a sensorsystem, in accordance with one aspect thereof, for monitoring soil andmaterial excavated from a buried waste containment site.

It is an additional object of the invention to provide such a sensorsystem, in accordance with another aspect thereof, for sensing physicalparameters of soil being excavated, during the excavation process.

It is a further object of the invention to provide such a sensor systemwhich may be readily installed at a buried waste containment sitesimultaneously with the installation of a containment barrier.

It is also an object of the invention to provide such a sensor system inwhich sensors may be installed and removed after the buried wastecontainment site is in place.

The above and other objects of the invention are realized in a specificillustrative embodiment of a sensor system for a buried wastecontainment site having a bottom wall barrier and/or sidewall barriers,for containing hazardous waste. The sensor system includes one or moresensor devices disposed in one or more of the barriers for detecting aphysical parameter either of the barrier itself or of the physicalcondition of the surrounding soils and buried waste, and for producing asignal representing the physical parameter detected. Also included is asignal processing device for receiving signals produced by the sensordevice and for developing information identifying the physical parameterdetected, either for sounding an alarm, displaying a graphicrepresentation of the physical parameter detected on a viewing screenand/or a hard copy printout, etc.

In accordance with one aspect of the invention, the sensor devicedisposed in one or more of the barriers comprises a strain or cracktransducer for detecting strain or cracking and thus possible leakagelocations in the barrier in which the transducer is disposed. Oneembodiment of such a transducer includes a grid of detecting elementsdisposed in the barriers to detect strains wherever they might occur.

In accordance with another aspect of the invention, one or more accesstubes are disposed in or below the barriers with at least one end of thetubes extending from the barriers to allow access thereinto. Sensordevices are then disposed in the access tube or tubes and coupled to thesignal processing device through the one end of the tubes. The accesstubes provide protection for the sensor device without inhibitingoperation thereof. Also, use of access tubes allows for selectiveremoval and deployment of a variety of sensors.

In accordance with still another aspect of the invention, the sensordevice is adapted to detect radiation that may be leaking or may havealready leaked through the barriers, and/or the presence of RCRA metals.Also, a sensor device may be provided to detect volatile organiccompounds using fiber optic spectroscopy deployed in the access tubes.

In another embodiment of the invention, conveyor apparatus is providedfor removing and carrying away excavated earthen material. Disposedabove the conveyor apparatus and above any material being carried by theconveyor apparatus is one or more sensor devices for detecting variousconditions and components of the material being carried. The sensordevice is coupled to a processing device for developing informationidentifying the condition or components detected by the sensor device,just as with the sensor device disposed in the containment barriersdescribed above.

In another aspect of the invention, sensor detectable tracers could beused to verify barrier integrity. Specifically, tracers could be placedwithin the barrier with sensors outside the barrier to determine whetherthe tracers have migrated through a breach in the barrier, or stayed inplace.

In a further aspect of the invention, sensors or sensor arrays areinstalled in or about a barrier simultaneously with the installation ofthe barrier. For example the sensors or sensor arrays could be disposedbetween layers of a multi-layer barrier as the barrier is beinginstalled in a trench dug for that purpose.

As indicated earlier, one approach to installing a containment barrieraround a waste site involves the use of high pressure jets shooting aslurry into soil surrounding the waste site. This is also known asgrouting, and typically involves a grouting beam or arm which carriesthe jets and which is moved along a locus to both remove soil andproduce the containment barrier with a mixture of slurry and soil. Inaccordance with an aspect of the present invention, a sensor or sensorsare disposed on the grouting arm to detect physical properties of thesoil through which the arm moves, to thus determine whether contaminantshave leaked from the waste site into the surrounding soil.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the inventionwill become apparent from a consideration of the subsequent detaileddescription presented in connection with the accompanying drawings inwhich:

FIG. 1 is a perspective view of a plot of ground contaminated byhazardous waste;

FIG. 2 is a perspective view of the plot of ground with the hazardouswaste contained by a protective ground barrier;

FIG. 3 is a side, schematic view of sensor apparatus positioned above aconveyor carrying excavated material, in accordance with the presentinvention;

FIG. 4 is a perspective view of a grid sensor system deployed in acontainment barrier, in accordance with the present invention;

FIG. 5 is a side, schematic view of a fiber optic strain/crack sensorsystem deployed in a containment barrier, in accordance with the presentinvention;

FIG. 6 is a schematic view of a gamma spectroscopy sensor systemsuitable for use in the present invention;

FIG. 7 is a side view of a barrier placement machine suitable forconstructing a multilayer underground barrier and for simultaneouslydeploying sensor devices in the barrier; and

FIG. 8 is a side, cross-sectional view, enlarged, of the multi-layerunderground barrier of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a typical waste site 11 is shown containingdrums 13 filled with hazardous waste, both on the surface 15 and buriedunder the ground 17. Contaminants 19, leaking from the drums 13,threaten to migrate into a water table 12, unless some type ofcontainment barrier can be provided.

One such containment barrier 21 is shown in FIG. 2 to include sidebarriers or walls 23 and a floor or horizontal barrier 29. The sidebarriers 23 may be made using conventional methods and interconnected tothe horizontal barrier 29. Additionally, the waste site 11 could becompletely encapsulated by forming an upper barrier cover (not shown)and interconnecting it with the side barriers 23 and front and rearbarriers 25 and 27 (front barriers 25 are shown in phantom line in FIG.2). The afore-cited co-pending patent application describes howcontainment barriers of the type described may be constructed usingapparatus such as that to next be briefly described.

FIG. 3 is a side, schematic view of one embodiment of excavated soilsensor and assay equipment, in accordance with the present invention.FIG. 3 shows a conveyor 710 on which excavated soil 700 (from a wastecontainment site) is being carried for ultimate deposit. Disposed abovethe conveyor 991 for detecting various physical parameters andcontaminants of the soil 700 are a gamma ray spectrometer 704, an X-rayfluorescence detector 708, and a hood 712 for collecting vapors risingfrom the soil 700 and passing the vapors to an analyzer 716. Disposedunder (or could be over) the upper section of the conveyor 991 is ascintillating fiber bundle 720 coupled to an optical-to-electricalconvertor 728. The gamma ray spectrometer 704, X-ray fluorescencedetector 708, analyzer 716 and optical-to-electrical converter 728 areall coupled to a monitor 732 for processing signals received from thevarious components shown for displaying information represented by thereceived signals or for taking other action.

The gamma ray spectrometer 704 is provided for making measurements ofthe energies of particles emitted by different radioactive sources inthe soil 700 to thereby distinguish among the sources and identify them.The gamma ray spectrometer 704 supplies signals to the monitor 732identifying the different sources of radioactivity, and the monitorprocesses these signals to provide a display, hard copy printout, orother indication to a user of what sources of radioactivity are presentin the soil 700. Gamma ray spectrometers are well known in the art.

The X-ray fluorescence detector 708 is provided for detecting thepresence of RCRA metals in the soil 700. The detector 708 suppliessignals to the monitor 732 indicating whether or not RCRA metals havebeen detected, and the monitor then develops a suitable display,printout, etc. This type of detection is well known.

The hood 712 collects whatever vapors may be emitted by the soil 700,but in particular volatile organic compounds, and these are supplied tothe analyzer 716. The analyzer 716 could include a variety of devicesfor detecting the presence of volatile organic compounds including anacousto-optic tunable filter (AOTF) infrared spectrometer or aFourier-transform infrared spectrometer. Either of these devices issuitable for detecting the presence of volatile organic compounds andboth are well known in the prior art. If volatile organic compounds aredetected by the analyzer 716, the analyzer supplies signals to themonitor 732 identifying the volatile organic compounds and thisinformation may then be displayed, provided on a hard copy printout,etc.

The scintillating fiber bundle 720 is provided to detect the presence ofradiation emanating from the soil 700 being conveyed on the conveyor991. The fiber bundle 720, in the presence of different types ofradiation, emits light of a characteristic frequency, and this light isthen supplied to the optical-to-electrical converter 728. There, thelight is converted to electrical signals for supply to the monitor 732,for producing a display or other indication of the nature of theradiation detected.

Scintillating fiber bundles illustratively may be made of polystyrenefibers, doped with fluorescent compounds that scintillate in response tovarious kinds of ionizing radiation. This radiation-inducedscintillation comprises the light supplied to the optical-to-electricalconverter 728 for conversion to electrical signals. Scintillating fiberbundles are commercially available.

The monitor 732 might, advantageously, be a conventional computer-baseddata acquisition and display system, such as a Dell PC with Pentiumprocessor.

The sensing and assaying discussed above is for soil excavated as aresult of installing a waste containment barrier, for example inaccordance with the method described in the afore-cited co-pendingpatent application. It is also desirable to monitor the barrier itselffor integrity and to determine whether leakage of contaminated materialthrough the barrier is taking place. FIG. 4 is a perspective view of agrid sensor system for monitoring the integrity of a waste containmentbarrier 800. In one embodiment, the grid sensor system includes a firstplurality of conductors 804 extending generally in parallel in onedirection through the barrier 800, and a second plurality of conductors808 extending also generally in parallel in another direction in thebarrier to intersect with the first plurality of conductors at an endwall 800 a and a bottom wall 800 b (and the other end wall not shown) ofthe barrier 800. Both ends of the first plurality of conductors 804 andof the second plurality of conductors 808 are gathered and routed to asignal source and processor 812. The signal source and processor 812supplies electrical signals to both sets of conductors 804 and 808,which have a predetermined characteristic impedance. The electricalsignals supplied to one end of the sets of conductors will then bereceived by the signal source and processor 812 from the other end. Anystrain, i.e., change in dimension, which takes place in the material ofthe barrier 800, for example, such as the development of cracks oropenings, will affect the conductors 804 and 808. The affect will begenerally to elongate the conductors where the strain occurs and thiswill result in a change in the characteristic impedance of the affectedconductors. If a strain, for example, occurs near an intersection of oneof the conductors 804 and one of the conductors 808, then thecharacteristic impedance of those two conductors could be read by thesignal source and processor 812 and that would locate the location ofthe strain as being near the intersection. The change in characteristicimpedance can be measured with electrical time domain reflectometry, awell-known measuring technique. Once the location or locations of strainare detected by the signal source and processor 812 (e.g., spectrumanalyzer), it signals a monitor 816 which develops an output identifyingthe location of the strain. The monitor 816 might advantageously be acomputer-based data acquisition system, as with the monitor 732 in FIG.3.

An alternative embodiment to the conductor grid described above fordetermining integrity of the barrier 800, is a grid of fiber opticstrands disposed in the barrier 800 in the same manner as are theconductors. Assume that the conductors 804 and 808 are simply replacedwith fiber optic strands (as shown in a side view in FIG. 5) and thatthe signal source and processor 812 provides light of a certainintensity and wavelength to one end of strands 804 and 808 and then thatthe signal source and processor receives from the corresponding oppositeends the light that has been transmitted through the strands. If achange in wavelength and/or intensity of the light in any of the strandsis detected by the signal source and processor 812, such changeindicates that strain or cracking has occurred in the barrier 800 at alocation near the affected strands. Thus, detecting a change in thewavelength and/or intensity of light in two or more intersecting strandswould indicate that the strain or cracking has occurred near thatintersection and this information could be supplied by the signal sourceand processor 812 to the monitor 816 for display or other disposition.For processing the received light, the signal source and processor 812might illustratively be a commercially available optic time domainreflectometer, or optical spectrum analyzer, interfaced to a personalcomputer.

The spacing between conductors 804 and 808 or between fiber opticstrands 804 or 808 could illustratively be about one foot. This wouldenable identification of the location of strain or cracks in the barrier100 to resolution of about six inches.

Although a grid of either conductors or fiber optic strands were shownand described for FIG. 4, it is also possible to detect the location ofa strain or crack occurring in a barrier by an array of wires or fiberoptic strands extending parallel to one another and just in onedirection. In particular, a strain or crack which affects a single wirecan be located using electrical time domain reflectometry in which awavelength shift in a signal applied to the wire indicates a strain orcracking in the barrier, as analyzed by a spectrum analyzer. Electricaltime domain reflectometry is a well-known operation. Similarly, thelocation of a crack or strain affecting a fiber optic strand could bedetermined by measuring a back-reflected signal (reflected from thecrack or strain in the fiber) of an optical pulse sent down the fiber,using optical time domain reflectometry.

FIG. 5 shows another embodiment of a fiber optic strain/crack sensorsystem embedded in a containment barrier made of grout.

FIG. 6 shows a side schematic view of another embodiment of the presentinvention in which hollow access tubes 604 are disposed in a containmentbarrier 600, with the tubes being placed into the barrier (or below)during emplacement of the barrier. The access tubes 604 are used todeploy, among others, radiation sensors, such as scintillating fiberbundles or thermoluminescent dosimeters, X-ray fluorescence sensors fordetecting the presence of RCRA metals, and/or a fiber-optic spectroscopysystem to detect volatile organic compounds. The access tubes 904 couldbe emplaced in the barrier 900 using a variety of known deploymentmethods. The access tubes 904 may be placed in the bottom wall of thebarrier and/or the sidewalls thereof.

FIG. 6 shows a specific embodiment of a sensor system carried in theaccess tube 604 to include scintillating fiber bundles 608 (best seen inthe enlarged view 612 of a section of the barrier 600 and tube 604). Thescintillating fiber bundles 608 were discussed earlier in connectionwith FIG. 3, and operate to emit light of different frequenciesdepending upon the type of radiation to which the fiber bundles areexposed. Fiber optic strands 616 are carried by the access tube 604 andcoupled to the scintillating fiber bundles so that light emitted by thefiber bundles when exposed to radiation is carried by the fiber opticstrands to a monitor 620. The monitor 620 would include anoptical-to-electrical convertor for converting the light to electricalsignals for processing by a signal processing circuit to developinformation identifying the type of radiation detected which informationcould then be provided to a user.

X-ray fluorescence sensors could also be deployed in the access tube604, for detecting the migration of RCRA metals through the barrier 600,in a manner similar to that discussed in connection with FIG. 3.Conductors would be coupled to the X-ray fluorescence sensors forcarrying signals to the monitor 620 for processing and display ofinformation relating to the presence of RCRA metals.

Fiber-coupled optical systems based upon Raman and/or fluorescencespectroscopies could also be deployed in access tubes in or around thebarrier to detect and identify volatiles permeating through thecontainment barrier and through perforations 628 in the tube 604. Suchsystems operate by transmitting an excitation signal from a laser to asample volume at the distal end of an optical fiber, or fiber bundle,and then sampling and analyzing the excited gas in the volume with asecond fiber. This signal is then returned to a spectrometer andanalyzed to determine the type and concentration of volatiles present.The systems can be multiplexed to obtain samples from multiple locationsbeneath the barrier. Using available microchip laser technology, thelaser itself can be fiber-optically coupled and placed in the accesstubes. A fiber optic spectroscopy sensor 624 at the distal end of afiber 626 is shown in the enlarged views 612 and 614 of the access tube604.

Two other types of sensor systems could utilize the tube 604 of FIG. 6including acoustic sensors and radar sensor systems. Acoustic sensorscould be used to determine barrier emplacement performance and to gatherinformation about waste pit contents. Typically, arrays of acoustictransmitters would be disposed in tubes extending through the bottomwall containment barrier, for transmitting acoustic signals upwardlythrough the waste pit contents. Arrays of acoustic receivers aredeployed on the surface or just under the surface at the top of thewaste pit for receiving transmitted acoustic signals. The acousticreceivers in effect measure the propagation of various seismic waves,such as pressure waves, shear waves, raleigh waves, etc. (through thewaste pit contents), such propagation depending upon the elasticproperties of the contents. The arrays of transmitters and arrays ofreceivers are coupled via control cables to signal source and processorequipment and monitors for processing the acoustic signals anddisplaying information determined from the sensors, in a manner similarto the systems discussed earlier.

A radar system could also be used to map barrier performance. With sucha system, transmitters could be deployed in the tubes extending in thebottom wall of a barrier containment system to transmit electromagneticwaves upwardly through the waste pit contents to electromagnetic wavereceivers deployed on or near the surface of the waste pit.Heterogeneities in the waste pit contents (e.g., different soils,objects, moisture content, etc.) have different electromagneticproperties, transmitting electromagnetic waves through the wastecontents and then receiving and mapping the transmitted signals willprovide data about the contents and the performance of the wastecontainment barrier in containing the contents. Of course, thetransmitter arrays and receiver arrays would be coupled by cables (ortelemetry devices) to signal source and processor equipment and monitorsfor displaying the data derived from the transmission and reception ofelectromagnetic waves through the waste pit contents.

Although the acoustic sensor system and radar system described abovewere defined as transmitting signals from the bottom of the waste pit upto the surface thereof, it is obvious that the transmitters could bearranged on one side of the waste pit, with receivers arranged on theopposite side and that the signals could be transmitted effectivelyhorizontally through the waste pit contents. In this case, thetransmitters would be deployed in access tubes located on one side ofthe waste pit, with receivers deployed in access tubes located on theother side of the waste pit.

A resistivity system might also be deployed in the tubes for measuringlong-term barrier performance. Such a system utilizes very low frequencyelectromagnetic fields (approaching the direct-current limit) to performdirect current resistivity measurements of the barrier contents. Anelectromagnetic wave transmitter would be deployed in a tube near thecenter of the waste pit at the bottom thereof, to transmit 360 degreesoutwardly, with receivers being located outside of the waste pit, eitherunderground or on the surface for receiving the transmitted waves. Theresistivity measurements would provide an indication of barrierintegrity such as imperfections, cracks and breaks.

With the arrangement of access tubes described in particular withrespect to FIG. 6, it is apparent that various sensors could be deployedin the access tubes simultaneously or one type sensor might be deployedfor data gathering at one point in time, then removed and another typesensor deployed in the access tubes for acquisition of different data.Since one or both ends of the access tubes would extend through thesurface of the ground, sensor arrays could easily be installed and laterremoved from the access tubes to make way for a different sensor array.

Advantageously, the access tubes 604 could be made of any flexible,electrically neutral material, and may be perforated, as shown at 628 inFIG. 6, to allow entry of VOC's for detection purposes. The access tubes604 could illustratively have an inside diameter of from 0.5 to 6inches. The spacing of the access tubes, advantageously, is about threefeet.

Another approach to monitoring barrier integrity involves the use of atracer system in which tracers are placed at various locations in thebarrier. The tracers could be dye, detectable by fluorescencespectroscopy, visual or chemical testing of samples of soil orgroundwater, or ferromagnetic material, detectable by magnetic sensors.The sensors would be placed outside the barrier in positions to detectmovement of the tracers and thus a possible breach in the integrity ofthe barrier.

Referring now to FIG. 7, there is shown an embodiment of a barrierplacement machine 220. The barrier placement machine 220 includes anoperator's cab 97, a cutting chain and grout injector assembly 333including cutter teeth 31 and discharge paddles 33, a grout receivingconveyor 959, a soil retaining shield traveling pan 953, a soilretaining shield consolidator 955, a side trench excavator 91, soilconveyor 933, and track mechanism 975 for moving the entire machine 220.The machine 220 is depicted in FIG. 7 in schematic form, and may includeall other components necessary for its operation, as understood by thoseof ordinary skill in the relevant field.

As the barrier placement machine 220 moves forward, a trench excavator91 digs a side trench shown in phantom line at 226. The trench excavator91 carries the excavated soil 984 up out of the ground and dumps it onthe trench excavator conveyor 991, which carries the soil backwardlyalong the machine 220. Grout or other suitable barrier forming materialis then placed within the side trench 226 by the soil retaining shieldtraveling pan 953 and the soil retaining shield consolidator 955, alongwith any other necessary grout injecting devices known to those ofordinary skill, to form the side barrier. The trench excavator conveyor991 dumps the soil 984 behind the barrier placement machine 220,refilling the side trench 226. Simultaneously, the cutting chain andgrout injector assembly 333 and soil conveyor 933 operate to excavateearthen material 985 from beneath the in-situ portion of earth 216without removing said in-situ portion, and discharges the soil 985 aboveground as shown in FIG. 7 where it lies conveniently accessible fortesting if desired.

The machine 220 further includes a barrier-forming means 953, 955 and224 attached to the excavating means 31, 33 and 91 for simultaneouslyforming a side barrier and a generally horizontal, multi-layer barrier228 (or could be a single-layer) within the generally horizontal trench222, said multi-layer barrier 228 having at least a first layer 202 anda second layer 204. This is further described in the afore-citedco-pending application.

Regarding the horizontal, multi-layer barrier 228, a horizontal barrierforming mechanism 224 is provided for forming at least a portion of thesecond layer 204 simultaneously with forming at least a portion of thefirst layer 202. More specifically, the horizontal barrier formingmechanism 224 includes: a first injector 232 for injecting a firstmaterial for forming the first layer 202 in the horizontal trench 222; amechanism for placing an intermediate shield 234 over the material forthe first layer 202; a second injector 236 for injecting a secondmaterial for forming the second layer 204 onto the intermediate shield234; and a frame 238 to which the intermediate shield 234 is attachedfor removing the intermediate shield 234 from between the first andsecond material forming the first and second layers 202 and 204. Theintermediate injectors 232 and 236 and, as an extension of the frame238, is advanced horizontally between the first and second layers 202and 204 as they are formed, as the track mechanism 975 advances themachine 220.

The first and second injectors 232 and 236 are contained within firstand second chambers 240 and 242, respectively. The intermediate shield234 thus operates as a carrying member coupled to the chambers 240 and242. The third, middle layer 212 begins a dispensable, pre-formed roll244 of barrier material that resides in a suitably sized trench 246. Theroll 244 of barrier material includes a first end 248. Any suitableattaching means known to those of ordinary skill in the art may be usedfor attaching the first end 248 of the roll 244 of barrier material tothe intermediate shield 234, such that barrier material is withdrawnfrom the dispensable roll 244 as the machine 220 advances. In suchmanner the roll of material 244, which might comprise a high performancematerial such as polyethylene or any suitable geo-textile membranematerial, is pulled between the first and second layers 202 and 204 asthe machine 220 advances. In this embodiment, the barrier material ofthe roll 244 preferably has sufficient strength to be pulled between thefirst and second layers 202 and 204 without substantial tearing.

FIG. 8 depicts another embodiment of a barrier placement approach inwhich a dispenser 250 comprises a pre-formed roll of barrier materialrotatably disposed between horizontal digging elements 31, 33 and thechambers 240, 242. The second injector 236 is positioned to inject thesecond layer 204 on top of an intermediate shield 234 a such that saidshield 34 separates the second layer 204 and the pre-formed layer 212 assaid second layer 204 and said pre-formed layer 212 are beingrespectively injected and dispensed. The intermediate shield 34 athereby operates as a retaining plate.

Various sensors 846 (FIG. 7) and 850 (FIG. 8), of the types described,may be disposed on the barrier material (geo-textile membrane) of therolls 244 and 250, respectively, for sensing barrier integrity,radiation, etc. In this manner, any desired sensor can be deployedbetween the first and second layers 202 and 204 by being incorporatedinto the membrane barrier material forming the roll 244 or the roll 250.In other words, the sensors 846 or 850 can be installed at the same timeas the barrier 228 is installed.

Referring again to FIG. 8, sensors 35 may be installed in the cuttingteeth 31 to detect characteristics of the soil being removed such asvolatile organic compounds (VOCs), heavy metals and radiation, todetermine if contamination has leaked from the waste site. The sensors35 might illustratively be comprised of scintillating fiber opticbundles, x-ray fluorescence sensors, or fiber-coupled optical systems,for transmitting signals to a receiver located, for example, on thesurface to indicate the soil characteristics being detected.

In a manner similar to sensors 35 on the cutting teeth 31, sensors couldbe mounted on a grouting beam or arm, such as those disclosed in theforesighted International Publication Numbers WO 94/19547 & WO 93/00483by Halliburton Nus Environmental Corp., for detecting soilcharacteristics of soil through which the grouting beam is moved to formthe containment barrier.

It is to be understood that the above-described arrangements are onlyillustrative of the application of the principles of the presentinvention. Numerous modifications and alternative arrangements may bedevised by those skilled in the art without departing from the spiritand scope of the present invention and the appended claims are intendedto cover such modifications and arrangements.

1. In an underground containment barrier excavating and emplacementapparatus having means for excavating earthen material from about aburied waste site, and conveyor means for carrying the excavatedmaterial outwardly of the apparatus, the improvement comprising a sensorsystem for sensing physical properties of the excavated materialincluding: sensing means disposed adjacent the conveyor means forsensing selected physical properties of the material carried by theconveyor means, and for producing signals identifying the sensedphysical properties, and signal processor means for processing saidsignals and for producing human perceivable representations of thephysical properties identified by the signals, and wherein the conveyormeans is positioned substantially below the surface of the earthgenerally adjacent the underground containment barrier.
 2. A sensorsystem as in claim 1 wherein said sensing means comprises a gamma rayspectrometer disposed above the conveyor means for detecting radiationemanating from the material on the conveyor means.
 3. A sensor system asin claim 1 wherein said sensing means comprises an X-ray fluorescencedetector disposed above the conveyor means for detecting the presence ofRCRA metals in the material on the conveyor means.
 4. A sensor system asin claim 1 wherein said sensing means comprises scintillating fiberbundle means disposed below/above the conveyor means for detectingradiation emanating from the material on the conveyor means.
 5. A sensorsystem as in claim 1 wherein said sensing means comprises anacousto-optic tunable filter disposed above the conveyor means fordetecting volatile organic compounds present in the material on theconveyor means.
 6. A sensor system as in claim 1 wherein said sensingmeans comprises a Fourier-transform infrared spectrometer disposed abovethe conveyor means for detecting volatile organic compounds present inthe material on the conveyor means.
 7. An underground containmentbarrier excavating and emplacement apparatus for excavating earthenmaterial from about a buried waste site, comprising: a conveyor forcarrying excavated material outwardly of the apparatus, the conveyorbeing positioned substantially below the surface of the earth generallyadjacent the underground containment barrier; and a sensor system forsensing physical properties of the excavated material, wherein thesensor system comprises: at least one sensor disposed adjacent theconveyor and configured for producing at least one signal representingat least one sensed physical property of excavated material carried bythe conveyor, and a signal processor configured for processing the atleast one signal and producing human perceivable representations of theat least one sensed physical property represented by the at least onesignal.
 8. The apparatus of claim 7, wherein the least one sensorcomprises a gamma ray spectrometer configured for detecting radiationemanating from the excavated material on the conveyor.
 9. The apparatusof claim 7, wherein the at least one sensor comprises an X-rayfluorescence detector configured for detecting at least one RCRA metalpresent in the excavated material on the conveyor.
 10. The apparatus ofclaim 7, wherein the at least one sensor comprises a scintillating fiberbundle configured for detecting radiation emanating from the excavatedmaterial on the conveyor.
 11. The apparatus of claim 7, wherein the atleast one sensor comprises an acousto-optic tunable filter configuredfor detecting at least one volatile organic compound present in theexcavated material on the conveyor.
 12. The apparatus of claim 7,wherein the at least one sensor comprises a Fourier-transform infraredspectrometer configured for detecting at least one volatile organiccompound present in the excavated material on the conveyor.
 13. Theapparatus of claim 7, wherein the at least one sensor comprises aplurality of sensors.
 14. The apparatus of claim 13, wherein theplurality of sensors comprise at least two sensors from the groupconsisting of a gamma ray spectrometer, an X-ray fluorescence detector,a scintillating fiber bundle, an acousto-optic tunable filter, and aFourier-transform infrared spectrometer.